Based on kernel version 4.7.2. Page generated on 2016-08-22 22:48 EST.

1An introduction to the videobuf layer
2Jonathan Corbet <corbet@lwn.net>
3Current as of 2.6.33
45The videobuf layer functions as a sort of glue layer between a V4L2 driver
6and user space. It handles the allocation and management of buffers for
7the storage of video frames. There is a set of functions which can be used
8to implement many of the standard POSIX I/O system calls, including read(),
9poll(), and, happily, mmap(). Another set of functions can be used to
10implement the bulk of the V4L2 ioctl() calls related to streaming I/O,
11including buffer allocation, queueing and dequeueing, and streaming
12control. Using videobuf imposes a few design decisions on the driver
13author, but the payback comes in the form of reduced code in the driver and
14a consistent implementation of the V4L2 user-space API.
1516Buffer types
1718Not all video devices use the same kind of buffers. In fact, there are (at
19least) three common variations:
2021 - Buffers which are scattered in both the physical and (kernel) virtual
22 address spaces. (Almost) all user-space buffers are like this, but it
23 makes great sense to allocate kernel-space buffers this way as well when
24 it is possible. Unfortunately, it is not always possible; working with
25 this kind of buffer normally requires hardware which can do
26 scatter/gather DMA operations.
2728 - Buffers which are physically scattered, but which are virtually
29 contiguous; buffers allocated with vmalloc(), in other words. These
30 buffers are just as hard to use for DMA operations, but they can be
31 useful in situations where DMA is not available but virtually-contiguous
32 buffers are convenient.
3334 - Buffers which are physically contiguous. Allocation of this kind of
35 buffer can be unreliable on fragmented systems, but simpler DMA
36 controllers cannot deal with anything else.
3738Videobuf can work with all three types of buffers, but the driver author
39must pick one at the outset and design the driver around that decision.
4041[It's worth noting that there's a fourth kind of buffer: "overlay" buffers
42which are located within the system's video memory. The overlay
43functionality is considered to be deprecated for most use, but it still
44shows up occasionally in system-on-chip drivers where the performance
45benefits merit the use of this technique. Overlay buffers can be handled
46as a form of scattered buffer, but there are very few implementations in
47the kernel and a description of this technique is currently beyond the
48scope of this document.]
4950Data structures, callbacks, and initialization
5152Depending on which type of buffers are being used, the driver should
53include one of the following files:
5455 <media/videobuf-dma-sg.h> /* Physically scattered */
56 <media/videobuf-vmalloc.h> /* vmalloc() buffers */
57 <media/videobuf-dma-contig.h> /* Physically contiguous */
5859The driver's data structure describing a V4L2 device should include a
60struct videobuf_queue instance for the management of the buffer queue,
61along with a list_head for the queue of available buffers. There will also
62need to be an interrupt-safe spinlock which is used to protect (at least)
63the queue.
6465The next step is to write four simple callbacks to help videobuf deal with
66the management of buffers:
6768 struct videobuf_queue_ops {
69 int (*buf_setup)(struct videobuf_queue *q,
70 unsigned int *count, unsigned int *size);
71 int (*buf_prepare)(struct videobuf_queue *q,
72 struct videobuf_buffer *vb,
73 enum v4l2_field field);
74 void (*buf_queue)(struct videobuf_queue *q,
75 struct videobuf_buffer *vb);
76 void (*buf_release)(struct videobuf_queue *q,
77 struct videobuf_buffer *vb);
78 };
7980buf_setup() is called early in the I/O process, when streaming is being
81initiated; its purpose is to tell videobuf about the I/O stream. The count
82parameter will be a suggested number of buffers to use; the driver should
83check it for rationality and adjust it if need be. As a practical rule, a
84minimum of two buffers are needed for proper streaming, and there is
85usually a maximum (which cannot exceed 32) which makes sense for each
86device. The size parameter should be set to the expected (maximum) size
87for each frame of data.
8889Each buffer (in the form of a struct videobuf_buffer pointer) will be
90passed to buf_prepare(), which should set the buffer's size, width, height,
91and field fields properly. If the buffer's state field is
92VIDEOBUF_NEEDS_INIT, the driver should pass it to:
9394 int videobuf_iolock(struct videobuf_queue* q, struct videobuf_buffer *vb,
95 struct v4l2_framebuffer *fbuf);
9697Among other things, this call will usually allocate memory for the buffer.
98Finally, the buf_prepare() function should set the buffer's state to
99VIDEOBUF_PREPARED.
100101When a buffer is queued for I/O, it is passed to buf_queue(), which should
102put it onto the driver's list of available buffers and set its state to
103VIDEOBUF_QUEUED. Note that this function is called with the queue spinlock
104held; if it tries to acquire it as well things will come to a screeching
105halt. Yes, this is the voice of experience. Note also that videobuf may
106wait on the first buffer in the queue; placing other buffers in front of it
107could again gum up the works. So use list_add_tail() to enqueue buffers.
108109Finally, buf_release() is called when a buffer is no longer intended to be
110used. The driver should ensure that there is no I/O active on the buffer,
111then pass it to the appropriate free routine(s):
112113 /* Scatter/gather drivers */
114 int videobuf_dma_unmap(struct videobuf_queue *q,
115 struct videobuf_dmabuf *dma);
116 int videobuf_dma_free(struct videobuf_dmabuf *dma);
117118 /* vmalloc drivers */
119 void videobuf_vmalloc_free (struct videobuf_buffer *buf);
120121 /* Contiguous drivers */
122 void videobuf_dma_contig_free(struct videobuf_queue *q,
123 struct videobuf_buffer *buf);
124125One way to ensure that a buffer is no longer under I/O is to pass it to:
126127 int videobuf_waiton(struct videobuf_buffer *vb, int non_blocking, int intr);
128129Here, vb is the buffer, non_blocking indicates whether non-blocking I/O
130should be used (it should be zero in the buf_release() case), and intr
131controls whether an interruptible wait is used.
132133File operations
134135At this point, much of the work is done; much of the rest is slipping
136videobuf calls into the implementation of the other driver callbacks. The
137first step is in the open() function, which must initialize the
138videobuf queue. The function to use depends on the type of buffer used:
139140 void videobuf_queue_sg_init(struct videobuf_queue *q,
141 struct videobuf_queue_ops *ops,
142 struct device *dev,
143 spinlock_t *irqlock,
144 enum v4l2_buf_type type,
145 enum v4l2_field field,
146 unsigned int msize,
147 void *priv);
148149 void videobuf_queue_vmalloc_init(struct videobuf_queue *q,
150 struct videobuf_queue_ops *ops,
151 struct device *dev,
152 spinlock_t *irqlock,
153 enum v4l2_buf_type type,
154 enum v4l2_field field,
155 unsigned int msize,
156 void *priv);
157158 void videobuf_queue_dma_contig_init(struct videobuf_queue *q,
159 struct videobuf_queue_ops *ops,
160 struct device *dev,
161 spinlock_t *irqlock,
162 enum v4l2_buf_type type,
163 enum v4l2_field field,
164 unsigned int msize,
165 void *priv);
166167In each case, the parameters are the same: q is the queue structure for the
168device, ops is the set of callbacks as described above, dev is the device
169structure for this video device, irqlock is an interrupt-safe spinlock to
170protect access to the data structures, type is the buffer type used by the
171device (cameras will use V4L2_BUF_TYPE_VIDEO_CAPTURE, for example), field
172describes which field is being captured (often V4L2_FIELD_NONE for
173progressive devices), msize is the size of any containing structure used
174around struct videobuf_buffer, and priv is a private data pointer which
175shows up in the priv_data field of struct videobuf_queue. Note that these
176are void functions which, evidently, are immune to failure.
177178V4L2 capture drivers can be written to support either of two APIs: the
179read() system call and the rather more complicated streaming mechanism. As
180a general rule, it is necessary to support both to ensure that all
181applications have a chance of working with the device. Videobuf makes it
182easy to do that with the same code. To implement read(), the driver need
183only make a call to one of:
184185 ssize_t videobuf_read_one(struct videobuf_queue *q,
186 char __user *data, size_t count,
187 loff_t *ppos, int nonblocking);
188189 ssize_t videobuf_read_stream(struct videobuf_queue *q,
190 char __user *data, size_t count,
191 loff_t *ppos, int vbihack, int nonblocking);
192193Either one of these functions will read frame data into data, returning the
194amount actually read; the difference is that videobuf_read_one() will only
195read a single frame, while videobuf_read_stream() will read multiple frames
196if they are needed to satisfy the count requested by the application. A
197typical driver read() implementation will start the capture engine, call
198one of the above functions, then stop the engine before returning (though a
199smarter implementation might leave the engine running for a little while in
200anticipation of another read() call happening in the near future).
201202The poll() function can usually be implemented with a direct call to:
203204 unsigned int videobuf_poll_stream(struct file *file,
205 struct videobuf_queue *q,
206 poll_table *wait);
207208Note that the actual wait queue eventually used will be the one associated
209with the first available buffer.
210211When streaming I/O is done to kernel-space buffers, the driver must support
212the mmap() system call to enable user space to access the data. In many
213V4L2 drivers, the often-complex mmap() implementation simplifies to a
214single call to:
215216 int videobuf_mmap_mapper(struct videobuf_queue *q,
217 struct vm_area_struct *vma);
218219Everything else is handled by the videobuf code.
220221The release() function requires two separate videobuf calls:
222223 void videobuf_stop(struct videobuf_queue *q);
224 int videobuf_mmap_free(struct videobuf_queue *q);
225226The call to videobuf_stop() terminates any I/O in progress - though it is
227still up to the driver to stop the capture engine. The call to
228videobuf_mmap_free() will ensure that all buffers have been unmapped; if
229so, they will all be passed to the buf_release() callback. If buffers
230remain mapped, videobuf_mmap_free() returns an error code instead. The
231purpose is clearly to cause the closing of the file descriptor to fail if
232buffers are still mapped, but every driver in the 2.6.32 kernel cheerfully
233ignores its return value.
234235ioctl() operations
236237The V4L2 API includes a very long list of driver callbacks to respond to
238the many ioctl() commands made available to user space. A number of these
239- those associated with streaming I/O - turn almost directly into videobuf
240calls. The relevant helper functions are:
241242 int videobuf_reqbufs(struct videobuf_queue *q,
243 struct v4l2_requestbuffers *req);
244 int videobuf_querybuf(struct videobuf_queue *q, struct v4l2_buffer *b);
245 int videobuf_qbuf(struct videobuf_queue *q, struct v4l2_buffer *b);
246 int videobuf_dqbuf(struct videobuf_queue *q, struct v4l2_buffer *b,
247 int nonblocking);
248 int videobuf_streamon(struct videobuf_queue *q);
249 int videobuf_streamoff(struct videobuf_queue *q);
250251So, for example, a VIDIOC_REQBUFS call turns into a call to the driver's
252vidioc_reqbufs() callback which, in turn, usually only needs to locate the
253proper struct videobuf_queue pointer and pass it to videobuf_reqbufs().
254These support functions can replace a great deal of buffer management
255boilerplate in a lot of V4L2 drivers.
256257The vidioc_streamon() and vidioc_streamoff() functions will be a bit more
258complex, of course, since they will also need to deal with starting and
259stopping the capture engine.
260261Buffer allocation
262263Thus far, we have talked about buffers, but have not looked at how they are
264allocated. The scatter/gather case is the most complex on this front. For
265allocation, the driver can leave buffer allocation entirely up to the
266videobuf layer; in this case, buffers will be allocated as anonymous
267user-space pages and will be very scattered indeed. If the application is
268using user-space buffers, no allocation is needed; the videobuf layer will
269take care of calling get_user_pages() and filling in the scatterlist array.
270271If the driver needs to do its own memory allocation, it should be done in
272the vidioc_reqbufs() function, *after* calling videobuf_reqbufs(). The
273first step is a call to:
274275 struct videobuf_dmabuf *videobuf_to_dma(struct videobuf_buffer *buf);
276277The returned videobuf_dmabuf structure (defined in
278<media/videobuf-dma-sg.h>) includes a couple of relevant fields:
279280 struct scatterlist *sglist;
281 int sglen;
282283The driver must allocate an appropriately-sized scatterlist array and
284populate it with pointers to the pieces of the allocated buffer; sglen
285should be set to the length of the array.
286287Drivers using the vmalloc() method need not (and cannot) concern themselves
288with buffer allocation at all; videobuf will handle those details. The
289same is normally true of contiguous-DMA drivers as well; videobuf will
290allocate the buffers (with dma_alloc_coherent()) when it sees fit. That
291means that these drivers may be trying to do high-order allocations at any
292time, an operation which is not always guaranteed to work. Some drivers
293play tricks by allocating DMA space at system boot time; videobuf does not
294currently play well with those drivers.
295296As of 2.6.31, contiguous-DMA drivers can work with a user-supplied buffer,
297as long as that buffer is physically contiguous. Normal user-space
298allocations will not meet that criterion, but buffers obtained from other
299kernel drivers, or those contained within huge pages, will work with these
300drivers.
301302Filling the buffers
303304The final part of a videobuf implementation has no direct callback - it's
305the portion of the code which actually puts frame data into the buffers,
306usually in response to interrupts from the device. For all types of
307drivers, this process works approximately as follows:
308309 - Obtain the next available buffer and make sure that somebody is actually
310 waiting for it.
311312 - Get a pointer to the memory and put video data there.
313314 - Mark the buffer as done and wake up the process waiting for it.
315316Step (1) above is done by looking at the driver-managed list_head structure
317- the one which is filled in the buf_queue() callback. Because starting
318the engine and enqueueing buffers are done in separate steps, it's possible
319for the engine to be running without any buffers available - in the
320vmalloc() case especially. So the driver should be prepared for the list
321to be empty. It is equally possible that nobody is yet interested in the
322buffer; the driver should not remove it from the list or fill it until a
323process is waiting on it. That test can be done by examining the buffer's
324done field (a wait_queue_head_t structure) with waitqueue_active().
325326A buffer's state should be set to VIDEOBUF_ACTIVE before being mapped for
327DMA; that ensures that the videobuf layer will not try to do anything with
328it while the device is transferring data.
329330For scatter/gather drivers, the needed memory pointers will be found in the
331scatterlist structure described above. Drivers using the vmalloc() method
332can get a memory pointer with:
333334 void *videobuf_to_vmalloc(struct videobuf_buffer *buf);
335336For contiguous DMA drivers, the function to use is:
337338 dma_addr_t videobuf_to_dma_contig(struct videobuf_buffer *buf);
339340The contiguous DMA API goes out of its way to hide the kernel-space address
341of the DMA buffer from drivers.
342343The final step is to set the size field of the relevant videobuf_buffer
344structure to the actual size of the captured image, set state to
345VIDEOBUF_DONE, then call wake_up() on the done queue. At this point, the
346buffer is owned by the videobuf layer and the driver should not touch it
347again.
348349Developers who are interested in more information can go into the relevant
350header files; there are a few low-level functions declared there which have
351not been talked about here. Also worthwhile is the vivi driver
352(drivers/media/platform/vivi.c), which is maintained as an example of how V4L2
353drivers should be written. Vivi only uses the vmalloc() API, but it's good
354enough to get started with. Note also that all of these calls are exported
355GPL-only, so they will not be available to non-GPL kernel modules.